The Doppler lidar method is the classic method for determining velocities by measuring backscattered or reflected laser light. The wind lidar method, for example, detects a laser signal that is backscattered by air molecules or aerosols. If the air molecules or aerosols are moving under the influence of wind, the backscattered or reflected laser signal is frequency-shifted as a function of the wind velocity. The resulting frequency difference or so-called Doppler frequency between the frequency of the emitted or reflected laser signal or radiation and the backscattered signal or radiation is measured and provides a direct measure of the line-of-sight (LOS) velocity of the wind, also known as radial wind velocity.
In the conventional Doppler lidar method the Doppler frequency is measured with the aid of a local oscillator (LO) for example in the form of an auxiliary laser and the LO signal is heterodyned onto the received signal. The resulting beat frequency corresponds to .DELTA..lambda. or Doppler frequency, which is the basis for determining the wind velocity.
A useful beat signal with this type of coherent detection, however, is generated only when a required coherence condition between the received signal and the LO signal is satisfied, i.e. when a constant phase relationship exists between the two signals. The coherence condition might not be satisfied due to various reasons. For example, it can be disturbed by depolarization or by polarization mismatch of both the emitted and the received signals. Further, the coherence condition may not be satisfied due to wave-front disturbances resulting from a poor quality of the optical components, misalignment (wave-front tilt), or misfocussing (wave-front bending). Interference effects in the backscattered signal by so-called "speckles" can also prevent satisfying the coherence condition. If the coherence condition is not satisfied, the usable signal recedes or diminishes rapidly or even disappears completely. A further disadvantage of the conventional Doppler lidar method is that, the shorter the wavelength, the more difficult it is to satisfy the coherence condition. This is disadvantageous because the backscattering in the atmosphere increases as the wavelength decreases.
Due to the difficulties of satisfying the coherence condition when using short wavelength signals, attempts have been made to replace the coherent detection method with a non-coherent method. One method that has been suggested is the so-called "edge" technique which uses a narrow band optical filter that is so constructed that the original wavelength of the laser lies precisely on the edge of the filter transmission curve. The filter then transmits precisely 50% of the received signal. If the laser frequency of the received beam or signal shifts in one direction, transmission increases; if the laser frequency shifts in the other direction, transmission decreases. The increase or decrease of the transmission indicates the direction and the magnitude of the Doppler frequency shift of the backscattered laser beam, relative to the emitted original laser beam. The wind velocity can then be determined based on the frequency shift.
The "edge" technique, however, runs into great difficulties when put into practice. For example: the filter band width must be extremely narrow in order to provide the desired velocity resolution of approximately 1 m/s in the visible or near infrared range. Moreover, relatively expensive equipment is required in order to precisely tune the filter edge and to provide a consistently stable match of the filter to the laser wavelength. Further, the evaluation and calibration of the absolute value of the filter transmission entail significant difficulties. Yet another drawback of the "edge" technique is that it can be used only with a stationary measuring platform. As soon as the measuring platform moves, the frequency of the backscattered signal shifts relative to or about the velocity of the measuring platform and, under unfavorable conditions, the backscattered signal falls outside the filter transmission curve. Thus, if the measuring platform moves, it is critical that its velocity be known very precisely and that a method be applied that will exactly tune or track the filter curve to the measuring platform velocity. For this reason, the "edge" technique according to the current state of the art appears to be impractical for use in aircraft or satellites.